The present invention relates to a method for checking the functional reliability of an image sensor having a plurality of light-sensitive pixels, wherein grey-scale values of at least one light-sensitive pixel are evaluated.
The invention furthermore relates to an apparatus, and in particular to an electronic camera, having an image sensor, which has a plurality of light-sensitive pixels, and further having an output unit, which provides grey-scale values from the light-sensitive pixels for read-out.
The invention is particularly related to a method and an electronic camera for use in a protective apparatus for safeguarding a hazardous area, such as the working area of an automated machine.
DE 100 17 333 A1 describes a protective apparatus for safeguarding a hazardous area and also a method for checking the functional reliability of the protective apparatus. The protective apparatus comprises an electronic camera which, by way of example, can record the hazardous area of a machine operating in an automated manner. The recorded image is evaluated by electronic image processing means in order to check whether, by way of example, a person is situated in the hazardous area. If appropriate, the machine is shut down.
In order to achieve the safeguarding desired with such a protective apparatus, it is necessary to ensure the functional reliability of the protective apparatus itself. For this reason, it is necessary to regularly check the functional reliability of the camera used in the protective apparatus.
In accordance with DE 100 17 333 A1, the image recorded by the camera is modified for the purpose of checking the functional reliability of the image sensor. The modified object image is then compared with a reference image comprising the expected modifications. If the modified image deviates from the reference image, a functional error of the image sensor is assumed. This method makes it possible to realize a protective apparatus which, inter alia, meets the high safety requirements of categories 3 and 4 of European Standard EN 954-1. In particular, it is possible to achieve an uninterrupted (in time) monitoring of the hazardous area and a checking of individual pixels. However, modification of the image requires mechanical and/or optical components which influence the beam path of the incident light. This makes the known protective apparatus relatively expensive.
EP 0 902 402 A2 discloses a method and an apparatus for optically monitoring a spatial area. In order to check the functional reliability of this apparatus, a camera is dazzled with a light-emitting diode. The image pattern then recorded by the camera is compared with a stored reference pattern. However, this method has the disadvantage that the camera is “blind” while the functional test is being carried out, which makes it impossible to effect uninterrupted monitoring. Moreover, this method cannot reliably check whether individual pixels or areas of the image sensor are defective.
EP 0 179 252 A2 discloses a protective apparatus having a camera which is based on the optical evaluation of colour differences. An operator who is to be protected wears clothing of different coloration from the hazardous machine. In order to check the functional reliability of the camera used for the monitoring, the presence of a colour pattern is continuously checked. However, these methods either do not allow to check, in a defined manner, individual pixels or areas of the image sensor used in the camera.